The majority of the weeds in the landscape now are winter annuals and will die back in early summer due to warmer weather. These weed seeds germinate in fall and early winter. As weeds mature, they become harder to kill and require more applications of herbicide to achieve results. Now, it may be challenging to get them under control with a post-emergent herbicide. Once we get into March, it is more economical to plan to manage them better next year. An ideal time to treat those winter weeds with post-emergent herbicide sprays is right around Thanksgiving.
However, now is the perfect time to prevent summer weeds like crabgrass, goosegrass, and chamberbitter from coming up in the lawn. In the following weeks, many things can be done to make sure your lawn is a beautiful green carpet this summer. The best defense against weeds is a thick, healthy lawn. If you haven’t had the soil in the lawn tested recently, get your soil analyzed through Clemson. The results will tell you the exact nutrients your lawn needs to maintain healthy growth. For more information on soil testing, see HGIC 1652, Soil Testing, and HGIC 1658, How to Complete a Soil Form for Sample Submission to the Agricultural Service Laboratory.
Culturally, there are many things homeowners can do to prevent weeds in their landscape. Make sure to keep a 3-inch layer of mulch in landscape beds. Keep your lawn cut at the upper end of the recommended mowing heights for your grass species. For more information on mowing height, see HGIC 1205, Mowing Lawns. Mowing at the higher height recommendation will help to shade out weed seeds. Most weed seeds need light to germinate. In fact, crabgrass is an indicator weed. Usually, if it is present, it signifies that the grass is too thin, mowed too low, or the soil is staying too moist. Never water the lawn more than once per week in spring or fall, as these are the times that most annual weed seeds germinate. Frequent watering of the lawn will keep the surface of the soil moist, which will encourage more weed seeds to germinate. Infrequent and deep watering will reduce the number of weeds that grow.
Pre-emergent herbicides provide very effective control when applied at the proper time and according to the label rate. Pre-emergent herbicides are used to prevent weed seedlings from growing, and most provide 60 days of control. There are various pre-emergent herbicides on the market, and most of them are safe on all warm-season and cool-season grasses. Most pre-emergent herbicides are effective at controlling annual grassy weeds and some broadleaf weeds. The broadleaf weeds controlled by pre-emergent herbicides vary by product. If you need help determining an effective product for your landscape, please see HGIC 2300 Grassy Weeds, HGIC 2310 Managing Weeds in Warm-Season Lawns, and HGIC 2309 Managing Weeds in Fescue Lawns.
Crabgrass is the first summer weed to start germinating. It will begin to germinate when the soil temperature reaches 57 . Since most of us do not track soil temp, a good rule of thumb is to apply pre-emergent herbicide once we receive several 70 days. Goosegrass usually begins to germinate a week after crabgrass. Most summer annual weeds begin to germinate later in April.
To achieve the best control with a pre-emergent herbicide application, split the recommended dose in half. Apply the first half to your lawn in a vertical direction. Then apply the second half of the product in a horizontal pattern. Applying the product in two different directions will guarantee uniform coverage of the lawn. Pre-emergent herbicides need to be watered-in with a half-inch of water to allow them to move through the soil and reach an effective depth to stop germinating weed seeds. Products need to be watered-in immediately, but at least within seven to ten days to prevent sunlight from breaking them down and losing effectiveness.
A note of caution for centipede lawns; most pre-emergent herbicides belong to the DNA chemical family, and these herbicides stop weeds from growing by preventing cells in the root system from elongating. This causes the roots to remain very small and ineffective at taking up water and nutrients. Centipede stolons or runners have shallow roots, and DNA pre-emergent herbicides can prevent centipede grass from successfully rooting into the soil. This has a long-term effect of keeping centipede lawns thin and allowing more weeds to thrive in the lawn. A better approach for centipede lawns to split the recommend rate of pre-emergent in half. Apply the first half early in the season and the second half of the application sixty days later to provide effective control of weeds while limiting the effect of clubbed roots on turf.